Science Experiments - Gamma-Ray Spectrometer

The Gamma-ray Spectrometer Experiment measured the composition of the lunar surface.

The Apollo Moon landings returned samples for study on Earth from only six locations on the Moon. In order to better understand the Moon's overall chemical composition, the Gamma-ray Spectrometer and the X-ray Fluorescence Spectrometer studied the composition of the Moon's surface from lunar orbit. These experiments were flown on both Apollo 15 and Apollo 16. The Gamma-ray Spectrometer was deployed on a 7.6-meter-long boom, which is visible in some Metric Camera photographs.

Some elements, such as uranium and thorium, are naturally radioactive and emit gamma-rays as part of their radioactivity. The bombardment of the lunar surface by galactic cosmic rays causes some other elements to emit gamma-rays. The Gamma-ray Spectrometer measured this radiation from lunar orbit to produce maps of the abundances of thorium, iron, and titanium on the lunar surface. The different elements were identified based on the energy of the gamma-rays that they emitted. The abundances of these elements were determined from the intensity of the gamma-rays (the greater the intensity of the gamma-rays, the greater the abundance of the element). Unlike the X-ray Fluorescence Spectrometer, which worked only in sunlight, the Gamma-ray Spectrometer functioned both in sunlight and in darkness. As a result, the Gamma-ray Spectrometer mapped the entire region of the Moon flown over by Apollo 15 and 16, or about 20% of the lunar surface in all. Features as small as 100 kilometers across can be distinguished in the resulting maps. Maps of the distribution of iron and titanium for most of the Moon's surface were later derived by analyzing visible and near-infrared images obtained by the Clementine spacecraft. In addition to studying the lunar surface, the Gamma-ray Spectrometer was also used on the return voyage to Earth to survey the sky for astronomical gamma-ray sources.

This experiment found high iron abundances over all mare regions and lower abundances elsewhere. Thorium and titanium abundances were also highest over mare regions, but these two elements varied considerably in abundance in different parts of the maria. More details about these measurements and their relationship to lunar rock compositions are presented in the following documents.

Gamma-Ray Spectrometer Iron Abundance

The figure above shows the iron abundance measured by the Apollo Gamma-Ray Spectrometer. Red is a high abundance, yellow and green are intermediate abundances, and blue and purple are low abundances. High iron abundances are found in all mare regions and lower abundances are found elsewhere. In combination with the low aluminum abundance found in the maria by the X-ray Fluorescence Spectrometer, this indicates that the maria are covered by basalt. This was known from laboratory analysis of samples obtained by Apollo missions that landed in mare regions, but the orbital geochemistry experiments allowed this knowledge to be extended to a larger segment of the Moon. Data are only available for portions of the Moon overflown by Apollo 15 and Apollo 16. Another map of iron abundance was produced using observations made by the Clementine spacecraft. The Clementine results cover almost the entire Moon and are based on the effects of iron on the Moon's near-infrared spectrum rather than on its gamma-ray emissions. (From Plate 10.2 of Lunar Sourcebook, Cambridge University Press, 1991.)

Gamma-ray Spectrometer Thorium Abundance

The figure above shows the thorium abundance measured by the Apollo Gamma-ray Spectrometer. Data are only available for portions of the Moon overflown by Apollo 15 and 16. Red is a high abundance, yellow and green are intermediate abundances, and blue and purple are low abundances. Mare regions on the western part of the lunar nearside (Mare Imbrium and Oceanus Procellarum) show relatively high thorium abundances. Mare regions on the eastern part of the lunar nearside have lower thorium abundances. The lowest thorium abundances are on the lunar farside. Samples from the western part of the Moon's nearside obtained by Apollo 12, 14, and 15 include a type of basalt known as KREEP. KREEP is a chemical acronym, denoting rocks that are high in potassium (denoted chemically as K), rare earth elements (REE) and phosphorus (P). Thorium behaves chemically as one of the rare earth elements, and the high thorium regions in the above map indicate regions where KREEP is likely to be present on the Moon's surface. In contrast, samples from mare regions on the eastern part of the Moon's nearside obtained by Apollo 11 and 17 do not include significant amounts of KREEP. The Gamma-ray Spectrometer results indicate that KREEP is not important in this region of the Moon. (From Plate 10.1 of Lunar Sourcebook, Cambridge University Press, 1991.)

Gamma-ray Spectrometer Titanium Abundance

The figure shows the titanium abundance measured by the Apollo Gamma-Ray Spectrometer. Red is a high abundance, yellow and green are intermediate abundances, and blue and purple are low abundances. Titanium on the Moon's surface occurs primarily in the mineral ilmenite. Titanium is very rare in the lunar highlands and of variable abundance in the lunar maria. High titanium abundances are found in Mare Serenitatis and Mare Tranquillitatis. Among the lunar samples returned to Earth, the highest titanium abundances were in fact observed in Apollo 11 and Apollo 17 samples from these maria, although some very low titanium basalts were also sampled by Apollo 17. Another high titanium area is in western Oceanus Procellarum, a region that was not sampled by Apollo. The Gamma-ray Spectrometer data indicates that other mare regions have lower Ti abundances. Samples returned by Apollo 12, 14, and 15 and by Luna 16 and 24 are consistent with this observation. Data is only available for portions of the Moon overflown by Apollo 15 and Apollo 16. Another map of titanium abundance was produced using observations made by the Clementine spacecraft. The Clementine results cover almost the entire Moon and are based on the effects of titanium on the Moon's visible and near-infrared spectrum rather than on its gamma-ray emissions. (From Plate 10.3 of Lunar Sourcebook, Cambridge University Press, 1991.)